1. Field of the Invention
The present invention relates to an electrophotographic or electrostatic image forming apparatus such as a copying machine or printer.
2. Description of the Related Art
FIG. 1 illustrates an example structure of a color laser printer as an image forming apparatus. As shown in FIG. 1, the color laser printer forms electrostatic latent images on photosensitive members in image forming units with light emitted based on image signals transmitted from a controller (not shown). The electrostatic latent images formed on the photosensitive members are developed. The visible images are transferred in a superposed manner onto an intermediate transfer member so as to form a color visible image. This color visible image is transferred onto a recording material 2 and is then fixed.
The image forming units are aligned and develop four colors (yellow (Y), magenta (M), cyan (C), and black (K)). Each image forming unit has a photosensitive member (5Y, 5M, 5C, or 5K). Each image forming unit has a charger (7Y, 7M, 7C, or 7K) serving as a primary charging unit, and a developer (8Y, 8M, 8C, or 8K). The color laser printer further includes toner cartridges (11Y, 11M, 11C, and 11K) for supplying toners, an intermediate transfer member 12, a paper feeding unit 1, primary transfer units (29Y, 29M, 29C, and 29K), a secondary transfer unit 9, and a fixing unit 13.
The photosensitive members (5Y, 5M, 5C, and 5K), the chargers (7Y, 7M, 7C, and 7K) serving as primary charging units, and the developing units (8Y, 8M, 8C, and 8K) are integrated in process cartridges (22Y, 22M, 22C, and 22K) that are detachable from the image forming apparatus main body.
The photosensitive drums (also referred to as photosensitive members) 5Y, 5M, 5C, and 5K each include an aluminum cylinder and an organic photoconductive layer applied to the periphery of the cylinder. The driving force of a driving motor (not shown) is transmitted to the photosensitive drums 5Y, 5M, 5C, and 5K to rotate them. The driving motor rotates the photosensitive drums 5Y, 5M, 5C, and 5K counterclockwise in the figure in accordance with image forming operation. The photosensitive drums 5Y, 5M, 5C, and 5K are irradiated with light emitted from scanner units 10Y, 10M, 10C, and 10K. The surfaces of the photosensitive drums 5Y, 5M, 5C, and 5K are selectively irradiated with light in accordance with image signals so that electrostatic latent images are formed.
The four chargers 7Y, 7M, 7C, and 7K serving as primary charging units charge the yellow (Y), magenta (M), cyan (C), and black (K) photosensitive members. Each charger includes a charging roller (also referred to as charging sleeve) 7YS, 7MS, 7CS, or 7KS.
The four developers 8Y, 8M, 8C, and 8K, which serve as developing units, perform development of yellow (Y), magenta (M), cyan (C), and black (K) to make the electrostatic latent images visible. Each developer includes a developing roller (also referred to as developing sleeve) 8YS, 8MS, 8CS, or 8KS. Each developer is detachable. The intermediate transfer member 12 is in contact with the photosensitive drums 5Y, 5M, 5C, and 5K. The intermediate transfer member 12 rotates clockwise in the figure during the image formation. A drive roller 18 drives the intermediate transfer member 12. The visible images on the rotating photosensitive drums 5Y, 5M, 5C, and 5K are transferred to the intermediate transfer member 12. A transfer roller (29a, 29b, 29c, 29d) is a member to transfer the visible image from the photosensitive drums to the intermediate transfer member 12. Each of the transfer rollers (29a, 29b, 29c, 29d) is positioned at the position facing each of plural drums (5Y, 5M, 5C, and 5K).
During the image formation, a below-described transfer roller 9a comes into contact with the intermediate transfer member 12, the recording material 2 being nipped and conveyed. The color visible image formed on the intermediate transfer member 12 by the image forming units is transferred onto the recording material 2. While the color visible image is transferred onto the recording material 2, the transfer roller 9a is in contact with the intermediate transfer member 12. At the end of the printing process, the transfer roller 9a moves to a position 9b. 
While conveying the recording material 2, the fixing unit 13 fixes the color visible image transferred on the recording material 2. The fixing unit 13 includes a fixing roller 14 that heats the recording material 2, and a pressing roller 15 that presses the recording material 2 against the fixing roller 14. The fixing roller 14 and pressing roller 15 are hollow and house heaters 16 and 17, respectively. The recording material 2 holding the color visible image is conveyed by the fixing roller 14 and pressing roller 15, and heated and pressed so that the toner is fixed on the surface. After the visible image is fixed, the image forming operation is ended by ejecting the recording material 2 onto a paper ejecting section.
The color laser printer detects the conveying position and monitors the conveying state with sensors 23, 24, 25, 26, and 19, a pre-fixing sensor 27, a fixing/ejecting sensor 20, and an ejecting sensor 28 in a recording material conveying path. The sensors 23 and 25 detect a recording material supplied from a lower cassette 1B. The sensors 24 and 26 detect a recording material supplied from an upper cassette 1A.
A cleaner 21 serving as a cleaning unit 21 removes toners remaining on the intermediate transfer member 12. After the color visible image formed on the intermediate transfer member 9 is transferred onto the recording material 2, the cleaner 21 removes toners remaining on the intermediate transfer member 9 and stores the toners in a cleaner container.
A color misregistration sensor 6 detects the color misregistration of the image formed on the intermediate transfer member 9. A density sensor 4 detects the density of the image. Based on the detection results of these sensors, the color misregistration and density are corrected.
FIG. 2 is a block diagram illustrating a system configuration of the image forming apparatus. A controller 201 is capable of two-way communication with a host computer 200 and an engine control unit 202 (arrows 222 and 220 in FIG. 2). The engine control unit 202 includes a CPU 211 in communication with an image processing GA 212, image control unit 213, fixing control unit 214, sheet conveying unit 215 and driving control unit 216. The CPU and image processing GA are further in interfaced with video interface unit 210 which is interfaced to the controller 201 via lines 220 and 221.
The controller 201 receives image information and printing conditions from the host computer 200. The controller 201 transmits a print reservation command to the engine control unit 202 to make a reservation for a printing operation based on the received printing conditions and printing information (paper feeding unit, recording material size, printing mode, and so forth) of each recording material. The controller 201 analyzes the received image information and converts the image information into bitmap data (printing data). At the end of the analysis of the image information, the controller part 201 transmits a print start command for instructing to start the printing operation and printing data to the engine control unit 202.
Receiving the print start command, the engine control unit 202 outputs a /TOP signal (arrow 221 in FIG. 2) and starts the paper feeding operation. The /TOP signal serves as the reference timing of outputting of an image signal to the first image forming unit. After temporarily stopping the fed recording material at the registration roller 3 (see FIG. 1), the engine control unit 202 refeeds the recording material from the position of the registration roller 3 when the toner image formed on the intermediate transfer member reaches the secondary transfer position.
This /TOP signal instructs to write an image on the photosensitive drum in the image forming unit. The engine control unit 202 outputs this /TOP signal, and a latent image is formed on the photosensitive drum.
In the above-described image forming apparatus, the distance (A in FIG. 1) from the image formation starting position (developing position) of the first (yellow) image forming unit to the secondary transfer position can be larger than the distance (B) from the paper feeding position to the secondary transfer position.
In this case, the number of images formed per unit time (hereinafter referred to as throughput) can be increased by forming a plurality of pages of images on the intermediate transfer member when a recording material is fed or when the recording material is refed from the registration roller 3. Therefore, control has been performed so that images are formed on the intermediate transfer member before a recording material is fed or refed.
The first image forming unit refers to the yellow image forming unit. The image formation starting position refers to the position where the development is started on the photosensitive drum. In FIG. 1, the image formation starting position corresponds to the position where a toner image is developed on the yellow photosensitive drum.
FIG. 3 is a timing chart in the case where a plurality of pages of images are formed on the intermediate transfer member to increase throughput. Throughput means the number of images formed per unit time.
Receiving a print start command from the controller 201, the engine control unit 202 performs a preparation operation for printing. After completion of the preparation operation, the engine control unit 202 outputs a /TOP signal for the first recording material and outputs /TOP signals for the subsequent recording materials so that the time interval between successive /TOP signals (311, 321, and 331) for the recording materials has a desired value. A predetermined time after outputting the /TOP signal for each recording material, the engine control unit 202 starts the paper feeding operation (312, 322, and 332). Based on the time points (313, 323, and 333) when the supplied recording materials reach the registration roller 3, the engine control unit 202 conveys the leading edge of each recording material to a desired position and temporarily stops the conveyance of the recording materials (314, 324, and 334). In addition, time points (316, 326, 336) are the timing when an end of the recording material passes sensor 19. In synchronization with the conveyance of the toner images formed on the intermediate transfer member, the engine control unit 202 resumes the conveyance of the recording materials (315, 325, and 335) and transfers the toner images onto the recording materials. When the refeeding of the first sheet is started, the image formation for the third sheet is started.
In order to perform the above-described operation, it is necessary to form images on the intermediate transfer member so that there is a suitable distance (spacing) between the images to be transferred onto successive sheets of recording material. During feeding of a series of sheets to the secondary transfer unit there must be some gap between the trailing edge of one sheet and the leading edge of the next sheet. However, the minimum interval between sheets arriving at the secondary transfer unit is mainly determined by the sheet size in the direction in which the sheets are conveyed. Therefore, the engine control unit 202 needs to know the size of recording material in advance.
In the case where the size of recording material is not known in advance, the size of the first fed recording material is detected with a sensor in the recording material conveying path. After the size of the first recording material is detected, paper feeding operations are performed at an interval based on the detected size of recording material. This is disclosed in Japanese Patent Laid-Open No. 2000-272781.
FIG. 4 is a timing chart showing the operation in the case where the distance (A in FIG. 1) from the image formation starting position in the first image forming unit to the secondary transfer position is larger than the distance (B in FIG. 1) from the paper feeding position to the secondary transfer position, and the size of recording material is initially unknown (undetermined).
The engine control unit 202 outputs the /TOP signal of the first sheet (411) and image formation for the first sheet is commenced at that time. The engine control unit 202 also starts a paper feeding operation (412) for the first sheet a predetermined time after outputting the /TOP signal for that sheet. Based on the time point (413) when the supplied recording material reaches the registration roller 3, the engine control unit 202 conveys the leading edge of the first sheet of recording material to a desired position and temporarily stops the conveyance of the recording material (414). After that, in synchronization with the conveyance of the toner image formed on the intermediate transfer member, the conveyance of the first sheet of recording material is resumed (415) so that the toner image is transferred onto the first sheet of recording material. At this time, the engine control unit 202 carries out a size detection operation to detect the length in the conveying direction of the first recording material (hereinafter also referred to as actual length of recording material) based on the time from when the leading edge of the first recording material reaches the registration sensor 19 to when the trailing edge of the recording material leaves the registration sensor 19. Thus, completion (416) of the size detection operation does not occur until the trailing edge of the first sheet has reached the registration sensor 19.
When the trailing edge of the first sheet of recording material leaves the registration sensor 19, the engine control unit 202 starts the printing operation of the second sheet and outputs the /TOP signal of the second sheet (421). The engine control unit 202 outputs the /TOP signals of the second and subsequent sheets at an interval according to the detected length of recording material so that an optimum throughput can be achieved. Reference numeral 422 denotes the start of feeding of the second recording material. Reference numeral 423 denotes the time point when the second recording material reaches the registration roller. Reference numerals 424 and 425 denote the timing of driving the registration roller for the second recording material. Reference numeral 431 denotes the /TOP signal of the third recording material. Reference numeral 432 denotes the start of feeding of the third recording material. Reference numeral 433 denotes the time point when the third recording material reaches the registration roller. Reference numerals 434 and 435 denote the timing of driving the registration roller for the third recording material.
However, because in this case the size of recording material is initially unknown, the engine control unit 202 has to delay the output of the /TOP signal of the second sheet until the detection of the sheet size is completed based on the first sheet (i.e., until the first sheet leaves the sensor 19). Therefore, compared to the case where the size of recording material is known in advance, the printing interval between the first and second sheet is larger. This degrades throughput and therefore performance of the image forming apparatus.
The larger the distance from the image formation starting position of the first image forming unit on the intermediate transfer member to the secondary transfer position, the larger the degradation in performance.